1. Field of the Invention
The present invention relates to an automatic transmission mounted on a vehicle and, more particularly, to a speed change control system for addressing the problem which occurs when, during one speed change operation, need for another speed change operation different therefrom is determined.
2. Description of the Related Art
In a vehicle having an automatic transmission, during a speed change operation for change from a first speed stage to a second speed stage, a command for a speed change to a speed stage different from the second speed stage is sometimes issued. For example, during a speed change from a first speed to a second speed, the accelerator pedal may be depressed suddenly ("kick-down"), thus generating an instruction signal to execute a shift to the first speed.
Conventionally, as described in Japanese Patent Application Laid-Open Sho No.61-48561 for example, even if an up-shift instruction is entered during a down-shift operation, the up-shift is inhibited until the down-shift operation is completed. As a result, a sufficient amount of oil is discharged from an accumulator of a high-speed stage frictional engagement unit. When the servo hydraulic pressure of the high-speed stage frictional engagement unit has decreased to a sufficiently low level, the up-shift operation is started. In this manner a large shock in speed change is prevented, which shift shock would otherwise result from the resumption of oil supply to the accumulator of the high-speed frictional engagement unit before oil is completely discharged, as in the case where an up-shift operation is started during a down-shift operation.
However, with the prior art system described above, since the up-shift operation (second speed change) is inhibited until the down-shift operation (first speed change) is completed, the start of the down-shift is delayed for an unsatisfactorily long period of time.